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                    <h1 class="description center-align post-title">SPI Flash Modes 介绍</h1>
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                <p>该文章翻译来源为 <a target="_blank" rel="noopener" href="https://github.com/espressif/esptool/wiki/SPI-Flash-Modes">https://github.com/espressif/esptool/wiki/SPI-Flash-Modes</a></p>
<p>它主要介绍了我们烧录 ESP8266 和 ESP32 时对 SPI 接入方式该如何选择，每次我看到什么 DIO，DOUT，DIO 和 QOUT 都一脸懵B不知道该选什么，网上好像也没有什么专门讲这个的文章，至到我找网上翻到官方的文档，所以这里把它记录并翻译过来供大家参考，如果翻译错了请到 <a target="_blank" rel="noopener" href="https://github.com/Caffreyfans/Caffreyfans.github.io.git">https://github.com/Caffreyfans/Caffreyfans.github.io.git</a> 提交 issues 指出问题，我会即时修改。</p>
<span id="more"></span>
<p>ESP8266 和 ESP32 支持 4 种不同的 SPI flash 接入模式：DIO，DOUT，DIO 和 QOUT。他们可以在使用 <code>esptool.py write_flash</code> 使用 <code>--flash_mode</code> 选项。</p>
<p>这些选择控制多少 I/O 引脚被来与 SPI 闪存芯片连接，传输数据，和用哪种 SPI 命令。</p>
<p>ESP8266 和 ESP32 在从当从 SPI 闪存芯片读或执行代码和数据时用这些命令。读取数据，然后内部缓存到芯片。</p>
<h2 id="总结"><a href="#总结" class="headerlink" title="总结"></a>总结</h2><p>按性能排序：</p>
<table>
<thead>
<tr>
<th>选项</th>
<th>模式名</th>
<th>引脚使用</th>
<th>速度(ESP8266 &amp; ESP32)</th>
</tr>
</thead>
<tbody><tr>
<td>qio</td>
<td>四路 I/O</td>
<td>4 个引脚用于地址和数据</td>
<td>最快</td>
</tr>
<tr>
<td>qout</td>
<td>四路输出</td>
<td>4 个引脚用于数据</td>
<td>比 <code>qio</code> 模式约慢 15%</td>
</tr>
<tr>
<td>dio</td>
<td>两路 I/O</td>
<td>2 个引脚用于地址和数据</td>
<td>比 <code>qio</code> 慢约 45%</td>
</tr>
<tr>
<td>dout</td>
<td>两路输出</td>
<td>2 个引脚用于数据</td>
<td>比 <code>qio</code> 约慢 50%</td>
</tr>
</tbody></table>
<p>一般来说，应该为你的设备选择最快的 flash 模式。但是不是所有设备都支持所有工作模式。详情见下列 FAQ 部分。</p>
<h2 id="工作模式描述"><a href="#工作模式描述" class="headerlink" title="工作模式描述"></a>工作模式描述</h2><h3 id="常规-SPI"><a href="#常规-SPI" class="headerlink" title="常规 SPI"></a>常规 SPI</h3><p>一个传统的“单独”SPI（串行外设接口）总线使用 4 个引脚进行通信：</p>
<ul>
<li>时钟线（CLK）</li>
<li>主出从进（MOSI）</li>
<li>主进从出（MISO)</li>
<li>片选（CS）<br><a target="_blank" rel="noopener" href="https://en.wikipedia.org/wiki/Serial_Peripheral_Interface">维基百科里有关于 SPI 的完整介绍</a>。</li>
</ul>
<p>所有这些信号都是单向的。在单独的 SPI 模式，数据通过MISO引脚从设备发送到主机，并通过MOSI引脚从主机发送到设备。</p>
<p>正常 SPI 的最大数据速率是时钟速率（以位为单位）-所以一个 40MHz 的时钟 = 40Mbits/每秒 = 5Mbytes/每秒。</p>
<h3 id="双路-SPI"><a href="#双路-SPI" class="headerlink" title="双路 SPI"></a>双路 SPI</h3><p>为了提升性能，SPI 闪存厂商引进了“双路 SPI”。在双路 SPI 模式下，MOSI 和 MISO 引脚都被同时用来读取或写数据，每个时种周期为 2 位。相对于单独 SPI，一些指令就有两倍速率。</p>
<p>在 <code>dout</code> 模式下，主机使用“双输出快速读取”（3BH）命令去读取数据。每个读取指令和读取地址都通过常规 SPI 发送到闪存芯片，但主机同时通过 MOSI 和 MISO 引脚以每个时钟周期 2 位的速率读取数据。与仅使用 MISO 读取数据的单个 SPI 相比，这传数据传输速率提高了一倍。</p>
<p>在 <code>dio</code> 模式下，主机使用“双 I/O 快速读取”（BBH）命令去读取数据。每个读取指令都通过常规 SPI 发送到闪存芯片，但地址通过 MOSI 和 MISO 引脚以每个时钟周期 2 位的速率发送到闪存芯片。在这之后，主机以“双输出快读取”相同的方式读取每个时钟具有 2 位的速率。</p>
<p>对于 ESP8266 和 ESP32，每次指令读取 32 个字节，并且 dio 模式大约快 dout 快 5%。</p>
<p>请查阅您的特定 SPI 闪存芯片的数据表，以确定它是否支持这两个命令的一个或两个。</p>
<h3 id="四路-SPI"><a href="#四路-SPI" class="headerlink" title="四路 SPI"></a>四路 SPI</h3><p>为了进一步提高 SPI 闪存数据传输的性能，SPI 闪存厂商引进了“四路 SPI”模式。这种模式增加两个附加引脚（否则用于闪存芯片 WP 信号和 HOLD 信号）用于数据传输。这是使两 SPI 的数据传输速率提高一倍。</p>
<p>并不是所有 flash 芯片都支持四路 SPI 模式，而且并不是所有 ESP8266 和 ESP32 芯片都有这些引脚连接到 SPI 闪存芯片。一些闪存芯片要求特殊的指令实现四路模式（见下方）。</p>
<p>在 <code>qout</code> 模式，主机使用“四输出快速读取”（6BH）指令读取数据。这种命令与“双路输出快速读取”相同，只是数据读取用 4 个引脚代替了 2 个引脚，也就是每个时钟 4 位的速率。这使得传输数据实际快“双路输出快速读取”一倍。</p>
<p>在 <code>qio</code> 模式，主机使用“四路 I/O 快速读取”（EBH）指令读取数据。这命令与“双路 I/O 快速读取”相同，每个地址和数据传输用 4 个引脚代替了 2 个引脚，也就是每个时钟周期 4 位的速率。这使得地址和数据传输速度都比“双路 I/O 快速读取”快一倍。</p>
<h2 id="常问问题"><a href="#常问问题" class="headerlink" title="常问问题"></a>常问问题</h2><h3 id="为什么我的乐鑫芯片或模组不能使用-qio-或-qout-模式？"><a href="#为什么我的乐鑫芯片或模组不能使用-qio-或-qout-模式？" class="headerlink" title="为什么我的乐鑫芯片或模组不能使用 qio 或 qout 模式？"></a>为什么我的乐鑫芯片或模组不能使用 qio 或 qout 模式？</h3><p>这通常是以下原因：</p>
<ul>
<li>SPI 闪存芯片上的 WP 和 HOLD 引脚没有与乐鑫上的 GPIO 引脚正确连接。这些引脚必须正确连接才能让 quad 模式工作，不是所有板子和模组都连接了它们的。</li>
<li>SPI 闪存芯片不支持 quad 模式。查看 闪存芯片的数据表看它是否支持。你可以直接看闪存芯片上面印的型号或者使用 esptool.py flash 命令。</li>
<li>此芯片型号未正确启用 Quad 模式。SPI 闪存不是标准的，每个厂商实施它们芯片的方式都不一样。大多数闪存芯片需要发送某些命令才能启用 Quad SPI 模式，这些命令会有所不同。对于 ESP8266 和 ESP32，这通常意味着芯片第一次以双路 SPI 模式启动然后软件（ESP32 的启动引导或者 ESP8266 的SDK）检测芯片类型并尝试使能四路 SPI 模式。如果具体的芯片类型不被软件支持那么它将无法进入四路模式。</li>
</ul>
<h3 id="为什么-qout-和-dout-模式正常但是-qio-和-dio-模式不正常？"><a href="#为什么-qout-和-dout-模式正常但是-qio-和-dio-模式不正常？" class="headerlink" title="为什么 qout 和 dout 模式正常但是 qio 和 dio 模式不正常？"></a>为什么 qout 和 dout 模式正常但是 qio 和 dio 模式不正常？</h3><p>一些 SPI 闪存芯片类型只支持“双路输出快速读取”和“四路输出快速读取”指令，而不是等效的“双路 I/O”和“四路 I/O”。</p>
<h3 id="与四路-SPI-相比，我的代码在双路-SPI-模式下的运行速度快一半吗？"><a href="#与四路-SPI-相比，我的代码在双路-SPI-模式下的运行速度快一半吗？" class="headerlink" title="与四路 SPI 相比，我的代码在双路 SPI 模式下的运行速度快一半吗？"></a>与四路 SPI 相比，我的代码在双路 SPI 模式下的运行速度快一半吗？</h3><p>不是。ESP8266 和 ESP32 都是从 flash 里直接执行代码，但是，由于从闪存读取速度很慢，因此数据会透明地缓存在 RAM 中。仅在发生高速缓存未命令时发送闪存读取指令。但是，使用双路 SPI 读取重新填充缓存的速度大约是四路 SPI 的一半。</p>
<p>如果你不能使用四路 SPI 模式，请确保你配置的最快 SPI 闪存时钟速率能够让你的板子或者模组可靠地工作。两路 I/O 模式下的 80Mhz 时钟比四路 I/O 模式下的 40HMz SPI 时钟快。</p>
<h3 id="闪存是如何与-ESP8266-或-ESP32-通信的？"><a href="#闪存是如何与-ESP8266-或-ESP32-通信的？" class="headerlink" title="闪存是如何与 ESP8266 或 ESP32 通信的？"></a>闪存是如何与 ESP8266 或 ESP32 通信的？</h3><p>启动引导 .bin 文件，被烧录在 SPI 闪存里，头部包含闪存速度，闪存工作模式，和其他元数据。初始主机模式由 ROM 代码确定，当它在重置后会读取到该头部。esptool.py 为 <code>-flash_mode</code> 传入参数当文件被烧录到 flash 里时会更新头部。</p>
<p>这仅确定用于从复位进行初始引导的模式。然后，在启动过程中，软件可能会不同涤配置闪存模式。</p>
<p>比如，在 ESP32 上，如果将 ESP-IDF 配置为 qio/qout 模式，则实际上以 dio/dout 模式刷新 IDF 软件引导程序。当 ROM 代码从闪存引导该引导加载程序时，引导加载程序软件会检查闪存芯片型号，并在其余的引导过程中启用正确的四路 SPI 模式。这是因为有很多不同的方法可以在不同的芯片型号上启用四路 SPI。</p>

                
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